This invention relates generally to air conditioning and refrigeration systems and more specifically to cryogenic refrigeration systems.
In previous cryogen based refrigeration systems, a controller using a fuzzy logic scheme controlled the system. While the fuzzy logic scheme is well suited to controlling a cryogen system, it takes a substantial amount of time to generate a prediction of the required motor speed. The prediction time that is required by the fuzzy logic system often allows the motor speed to oscillate. On occasion, the motor speed may oscillate between 1400 revolutions-per-minute (xe2x80x9cRPMxe2x80x9d) and 1600 RPM, thus bringing the temperature of the conditioned space to an undesired range, and consequently damaging the load. The oscillation may sometimes even cause instability of the system. Furthermore, cryogen systems often have several modes of operation such as Cool, Heat, or Defrost, and also user-programmable control is preferred. However, the current fuzzy logic controllers evaluate system sensors to determine which mode to implement with little or no user input. Therefore, a user-programmable control system that regulates the motor speed in a manageable fashion would be welcomed by users of such systems.
According to the present invention, a method of temperature control in a cryogenic system, wherein the system includes a cryogen tank, and wherein the cryogen tank contains a cryogen, includes providing a motor speed sensor, the motor speed sensor being operatively coupled to a proportional-integral-derivative controller, the motor speed sensor determining a motor speed and sending the motor speed to the proportional-integral-derivative controller, providing a pressure sensor in the cryogen, the pressure sensor being operatively coupled to the proportional-integral-derivative controller, the pressure sensor determining a pressure at an end of an evaporator coil, and sending the pressure to the proportional-integral-derivative controller, providing a temperature sensor in the conditioned space, the temperature sensor being operatively coupled to the proportional-integral-derivative controller, the temperature sensor measuring a temperature within the conditioned space and sending the temperature to the proportional-integral-derivative controller, providing a deprived integral region in a proportional band to the proportional-integral-derivative controller when the motor speed is close to a motor speed set point, and generating an overriding control signal at the proportional-integral-derivative controller when the temperature and the pressure are beyond a temperature set point and a pressure set point.
In another aspect of the invention, a method of controlling a cryogenic temperature system, wherein the cryogenic system uses a proportional-integral-derivative control, controls the temperature within a conditioned space, and includes a cryogenic tank, includes determining a motor speed, determining a pressure of a cryogen, determining a plurality of temperatures inside the conditioned space, determining a plurality of temperatures out side the conditioned space, determining a new motor speed based on the motor speed, the pressure, the temperatures, and a plurality of predetermined temperature and pressure tables, and actuating the motor based on the new motor speed.
In yet another aspect of the present invention, a method of conserving a heat absorbing liquid in a cryogenic temperature control system, wherein the system includes a controller and a motor, and wherein the controller adjusts a motor speed, includes setting a target motor speed, averaging the motor speed over a predetermined amount of time after the system has entered a temperature controlling mode, regulating the heat absorbing liquid after the predetermined amount of time, resetting the target motor speed to a new target motor speed if the average motor speed is less than or equal to a predetermined speed below the target motor speed, the new target motor speed being set below the average motor speed by a predetermined motor speed value, and adjusting the motor speed such that the motor speed approaches the new target motor speed for a second predetermined amount of time.
In still another aspect of the present invention, a cryogenic temperature control system includes a conditioned space containing a gas and a load, the gas having a gas heat and thereby also having a temperature, a heat exchanger in the conditioned space, the heat exchanger having a heat absorbing liquid, and the heat absorbing liquid absorbing the gas heat within the conditioned space thereby lowering the temperature within the conditioned space, a heat source, the heat source releasing heat into the conditioned space thereby increasing the temperature within the conditioned space, a fan adjacent to the heat source and the heat exchanger, the fan circulating the gas in the conditioned space thereby having a fan speed, a temperature sensor determining a temperature within the conditioned space, a pressure sensor determining a cryogenic pressure at an end of an evaporator coil, and a controller operatively coupled to the fan, the temperature sensor, the pressure sensor, the heat exchanger, and the heat source, the controller receiving the temperature from the temperature sensor, receiving the pressure from the pressure sensor, adjusting the fan speed within the proportional band based on the temperature, the pressure, and the fan speed.
The controller of the present invention uses a proportional-integral-derivative (xe2x80x9cPIDxe2x80x9d) approach coupled with a xe2x80x9cwrapperxe2x80x9d program. The wrapper program evaluates the status of the refrigeration system, the environmental conditions, and user inputs to determine how the system should operate and in which mode the system should operate. This allows the user to quickly and easily override the program if desired. For example, a system installed on a truck may require quiet operation when passing through residential neighborhoods. The vapor motor fan is likely the largest producer of noise. A classical system using fuzzy logic would determine the fan speed based on the needs of the system. In the present invention, the user can set a lower vapor motor speed to provide quiet operation, and the system will compensate by adjusting other parameters such as cryogen flow.
A cryogen system controllable by the present method includes a micro-processor based controller, a cryogen tank for storage of liquid cryogen, a heat exchanger or evaporator, a heat exchanger fan driven by a vapor motor, a second heat exchanger for heating cryogen, and a heat source. In addition, a system uses valves and sensors throughout the system to control the flow of the cryogen and to monitor system parameters such as temperature at various points within the system and the CO2 pressure. Cryogen refrigeration systems generally use carbon dioxide (xe2x80x9cCO2xe2x80x9d) or nitrogen (xe2x80x9cN2xe2x80x9d) as the cryogenic fluid, however other fluids can be used.
The control method of the present system allows for the use of a cryogen based refrigeration system in one of several modes. These modes include 1) Heat, 2) Defrost, 3) Cool, 4) Null, and 5) Quench. While these active modes are included in the proposed method, the addition of other modes is contemplated by the present invention. Each of these modes except Null mode uses a PID (Proportional, Integral, Derivative) control method to control the fan speed within the system. In addition to these modes, the system includes several protection algorithms. These protection algorithms include 1) two ambient lockouts, 2) Top Freeze Protection, 3) Superheat Protection, and 4) CO2 Saver. While these algorithms are included in the contemplated system, the system is in no way limited to these alone.
To determine which mode the cryogen refrigeration system should be operating in at any given instant, a wrapper program or State Machine Program (xe2x80x9cSMPxe2x80x9d) is utilized. The SMP checks the ambient lockout status, and performs special functions for timer and flag initialization. The checks are performed based on the current state of the system, the previous state, and the anticipated future state of the system. If no ambient lockout exists and all special functions, timers, and flags are clear, the SMP chooses one of the five active modes contemplated at present in which the system can operate.
To understand the function performed by the SMP, a brief explanation of the different modes and lockouts is necessary. The system uses two ambient lockouts, a Heat Lockout and a Cool Lockout. Ambient lockouts are used to conserve cryogen. The Heat Lockout prevents the system from running in Heat mode when there is a call for heat and the ambient air temperature is above a preset value. The use of warm ambient air to provide the necessary heat rather than heating the cryogen or the return air from the cooled space reduces cryogen use. The Cold Lockout operates in a similar manner. When cooling is required and the ambient air temperature is below a preset value, ambient air is used to cool the cooled space rather than cryogen. If a lockout is present, the system cannot enter the corresponding Heat or Cool mode. The unit will remain in a Null mode.
Top Freeze Protection is initiated when the desired temperature within the cooled area is set to 32xc2x0 F. or higher. When the desired temperature is above 32xc2x0 F. the system under certain circumstances will admit cooling air that is colder than 32xc2x0 F. This cold air, falling on the cooled goods, can cause freezing of the outside portions of the goods. This is an undesirable effect. If the conditions that cause top freeze are present, the system will disengage the active control of the temperature and enter Top Freeze Protection. In Top Freeze Protection, the expansion valve is closed to reduce the cooling capacity of the system and prevent the discharge air from cooling to a temperature below 32xc2x0 F. The discharge air flows into the cooled space to provide cooling.
Superheat protection is used to minimize the possibility of flooding the vapor motor with liquid cryogen. The SMP monitors the evaporator coil conditions to assure that a preset amount of superheat exists within the cryogen vapor. When enough superheat does not exist, the SMP will engage Superheat Protection and disengage active temperature control. To determine the amount of superheat present, evaporator temperatures and pressures are measured. In one embodiment, the amount of superheat is calculated by comparing the evaporator coil temperature to the saturation temperature of the cryogen at the pressure measured at the outlet of the evaporator coil. This calculation uses several temperature and pressure sensors in conjunction with a known calculation to determine the amount of superheat. If the amount is below a preset and user adjustable value, the expansion valve is closed a preset percentagexe2x80x94in the preferred embodiment, 10%. The process is repeated periodically until the amount of superheat is acceptable or the valve reaches a certain preset positionxe2x80x94in the preferred embodiment, 30% open. In addition to monitoring the degree of superheat, the system monitors the condition of the sensors. Should one of the necessary sensors fail, the system is programmed to use alternative sensors or to switch into certain protective modes. For example, should the evaporator coil average temperature sensor fail, the system automatically switches into Superheat Protection mode to protect the vapor motor from potential damage. Many other methods are possible for calculating superheat, thus allowing the use of many different sensor arrangements.
CO2 Saver functions as an optimizer to the PID control algorithm. CO2 Saver is initiated when the system has been in a temperature controlling state for a predetermined period of time, but has been unable to reach the desired fan set point speed. Cryogen pressure is used to drive the vapor motor. Eventually, the cryogen tank will empty to a point at which the cryogen will not be capable of generating the pressure necessary to drive the vapor motor at the desired speed. The PID controller, in an effort to reach the fan speed set point, will force the valve to its full open position, wasting cryogen while still not achieving the desired speed. Under these conditions, the CO2 Saver will reduce the fan speed set point by a pre-selected value and allow the system to settle. If the fan speed is still below the set point speed, the process will be repeated. The fan speed set point will again be reduced and the system allowed to settle. The process will continue until the fan speed is approximately equal to the fan speed set point and the PID controller is controlling the system. This also happens when the unit is running at xe2x88x9220xc2x0 F. and the vapor motor is not as efficient as it is at +35xc2x0 F.
The Defrost mode is used when frost and ice has built up on the evaporator coil reducing the efficiency and performance of the unit. This mode can only be entered on demand and with user intervention. Once started, Defrost mode continues until a timer signals completion, the evaporator temperatures reach a preset value, or the door to the cooled space is open.
When none of the protective algorithms are operating, the SMP selects one of the five primary states, namely Heat, Cool, Defrost, Null, or Quench. Quench mode can only be selected following a Heat mode or a Defrost mode and is intended to prevent overheating of the heater and the evaporator coil. As such, it is not a true active control mode.
Heat, Cool, and Defrost are all active control states in that the PID controller is maintaining the speed of the fan motor at a desired set point. The set point is a function of the specific mode entered and is adjustable by the user. For example, in the preferred embodiment, if the Heat mode is selected, the PID controller will attempt to maintain the fan speed at 850 RPM. If the Cool mode is selected, two different speeds are possible, 1600 RPM, or 1200 RPM. The slower speed is used when conservation of cryogen is critical or noise abatement is required.
The Null mode is the mode in which the system spends a majority of time. In this mode the system temperatures are monitored and the system is maintained in a ready state in anticipation of returning to one of the active control modes of Heat, Cool, or Defrost.
The present system uses the SMP to determine which mode to operate in rather than a complex fuzzy logic scheme. This allows for the use of simpler processors, programs, and systems while still achieving the same level of control. The SMP uses inputs such as the cooled space temperature, ambient air temperature and desired temperature set points to determine which active control mode (Heat, Cool, Null, Quench, or Defrost) the system should be operating in. The SMP will also evaluate the input conditions to determine if the active mode should be overridden by ambient lockouts, Top Freeze Protection, Superheat Protection, or the CO2 Saver. If an override is warranted, the active control mode will not be initiated or will be discontinued, and the overriding algorithm will control the system operation. If no override is present, the system will enter one of the active control modes and control the fan speed at the desired set point value for that given mode. If no heating or cooling is necessary, the system will enter Null mode. While only a few modes are discussed, it is contemplated that the SMP could be utilized with additional modes not discussed and therefore should not be limited to only those modes. In addition, the override modes described are not the only overrides the SMP is capable of evaluating, others could be added as necessary for the given cryogen refrigeration system.